Morphogenesis, the progressive shaping of tissues, organs, and other anatomical structures in the embryo, is a central problem in developmental biology. Although recent years have seen tremendous progress in elucidating mechanisms of gene regulation during development, our understanding of the connection between gene expression and embryonic form is less complete. Morphogenesis has been challenging to study because it involves the coordination of many different cellular processes on the part of large numbers of embryonic cells. An understanding of morphogenesis will require an integration of cellular and molecular analyses; i.e., it will require relating molecular changes inside individual cells, such as changing patterns of gene expression or post-translational regulation of protein activity, with dynamic behaviors at the cellular level and even the supracellular properties of cell populations. This proposal focuses on three key morphogenetic processes common to all multicellular animals: 1) directional migration of embryonic cells, 2) cell rearrangements within epithelial sheets, and 3) regulation of the size of anatomical structures. These problems will be addressed in the optically clear, manipulable sea urchin embryo. 4-D fluorescence microscopy is to be used to analyze cell movements in vivo. In addition, experiments are proposed that build on recent biochemical studies in the PI's laboratory demonstrating a role for a specific extracellular matrix determinant in epithelial cell rearrangements during gastrulation. The problem of size regulation will be analyzed with respect to the skeletal system, using embryo manipulation, cell-type specific molecular markers, and an in vitro skeletogenesis system.